The present invention relates to an aperture grille used for cathode ray tube in color television set or color display device, and in particular to an aperture grille of lightweight design.
In a cathode ray tube of a color television set or a color display device, various types of shadow masks and aperture grilles having slit-like apertures are used so that electron beam is irradiated to a predetermined fluorescent material.
An aperture grille has a number of slit-like apertures. In order to accurately maintain its shape, it is attached on a frame body made of cast iron under high tension, and it is then mounted on a cathode ray tube.
The larger the cathode ray tube is designed, the heavier the weight of the aperture grille becomes. Even when the weight of the aperture grille itself is not increased very much, the tension required for mounting is increased, and it is unavoidable that the increased weight of the frame body with the aperture grille mounted on it leads to heavier weight of the cathode ray tube.
To solve this problem, thin metal material is used to achieve more lightweight design. FIGS. 4(A) and 4(b) show an example of a conventional type aperture grille. FIG. 4(A) is to explain the aperture grille, and FIG. 4(B) represents a frame body for mounting the aperture grille.
An aperture grille 1 comprises slits 2 designed in parallel to a fluorescent material arranged on a fluorescent surface, and grids 3. An upper end 4 and a lower end 5 of the aperture grille 1 are attached on an upper frame body 7 and a lower frame body 8 of a frame body 6 by means such as welding. When the aperture grille is mounted on the frame body, tension is applied on the aperture grille. For this reason, welding is performed under the condition that a force is applied in such manner that a distance between the upper frame body 7 and the lower frame body 8 is decreased.
Although the upper frame body 7 and the lower frame body 8 are made of a material with high rigidity, it is unavoidable that deflection occurs on the upper frame body 7 and the lower frame body 8. Deflection is more likely to occur at the central portion 7B of the upper frame body than at an end 7A of the upper frame body, and it is unavoidable that the distance between the central portion of the upper frame body and the lower frame body is reduced. As a result, tension between the upper frame body and the lower frame body is unavoidably turned to uneven.
Tensile strength applied on each grid of the aperture grille mounted on the frame body is proportional to resilient force applied on the aperture grille. Similarly to the distribution of resilient force, tensile strength is lower at the center and higher at both ends.
With the increasing trend to produce the cathode ray tube in larger size and with finer accuracy, thin metal material is used in the production. With the distribution of tensile strength as described above, problems arise such as rupture of grid on the peripheral portion of the aperture grille, or variation of elongation caused by heat.
To solve the above problems, the present applicant previously filed JP-A-5-174707, which discloses that the aperture grille is produced in lightweight design, weight of each grid of the aperture grille is made heavier at peripheral portion than at the central portion, and portions other than the portion with apertures are designed thinner when it is produced by etching. When aperture grille is produced by this method, it is necessary to design a resist pattern suitable for the thin portion and etching condition must be adjusted to suit the formation of the thin portion as desired.